For almost the entire history of the subject of superconductivity, ferromagnetism and superconductivity were thought to be incompatible. That is, if a material was found to be ferromagnetic, it was believed that it would not become superconducting. However, very recently a small number of materials have been found in which superconductivity and ferromagnetism coexist. These experimental discoveries have given rise to a number of puzzles, and there is thus much current interest in trying to understand why these materials behave as they do. The current situation is summarized well in an excellent article in Physics Today, Vol. 54, No. 9, p. 16, September 2001. Our group is working actively in this area – see the article M. B. Walker and K. V. Samokhin, Phys. Rev. Lett. 88, 207001-1 (2002). .
Immediately after the discovery of superconductivity in strontium ruthenate, this material became a subject of strong interest as a result of its structural similarity to the high temperature superconductors, and as a result of the unusual properties of its superconducting state. For example, the electrons forming the Cooper pairs in this superconductor are believed to form pairs with their spins parallel to one another, rather than antiparallel to one another as in conventional superconductors. An excellent tutorial introduction to the fascinating properties of strontium ruthenate superconductors can be found in Physics Today, Vol. 54, No. 1, p. 42, January 2001. One of the best ways of experimentally exploring the nature of the superconducting state of these materials is the study of the velocity and attenuation of sound waves propagated in the superconductor. The original experimental results produced a number of surprises which we have studied (e.g. see M. B. Walker, M. F. Smith, K. V. Samokhin, Phys. Rev. B 65, 014517 (2002).
In 1986 the discovery of high temperature superconductivity captured the interest and attention of the worldwide physics community. Today, interest in this subject remains intense, and we are actively working in this area. One of the problems recently resolved by our group was the understanding of the unusual way in which the excited electrons in the superconducting state of high temperature superconductors contribute to the electrial current produced in the superconductor by microwave electrical fields [see M. F. Smith and M. B. Walker, Phys. Rev. B 67, 214509 (2003)]. Another very interesting area that we have been involved in is the study of the symmetry of the superconducting state by making use of the Josephson effect (for a review see C C Tsuei and J R Kirtley, Rev. Mod. Phys. 72, 969 (2000).